Methods for dyeing keratinous fibers such as hair have been widely sought. Dyeing involves contacting the material to be colored with a solution or formulation containing the dye substance. However, problems are encountered with certain dye substances. Some dye substances are insoluble in the dyeing solution or formulation. Problems also arise if the dye substance has an insufficient temperature stability to be stored for extended periods of time. In such cases, practical application of such a dye substance is difficult, even if the coloration properties are otherwise highly desirable.
The inventors have now discovered, surprisingly and unexpectedly that pro-dyes of direct dyes having an enzymatically-labile functionality, particularly the derivatives of direct dyes of formula (I) defined below, referred to herein as pro-dyes, can in presence of a suitable enzyme dye the keratinous fibers. The enzyme cleaves the enzymatically-labile functionality, e.g. the glutaramide functionality, liberating the direct dye, which then colors the fiber.
As used herein, “direct dye” means a chemical composition that is capable of dyeing or coloring a dye-susceptible material without further chemical modification. As used herein, “pro-dye” means a derivative of a direct dye with a enzymatically-labile functionality, and which is converted into a direct dye in the presence of a suitable enzyme that can catalyze a reaction with said enzymatically-labile functionality. As used herein, “enzymatically-labile functionality” means a chemical functional group that can be reacted, cleaved, or modified in the presence of a suitable enzyme, e.g. the glutaramide group.